The present invention relates to a high strength and high corrosion resistance stainless steel, which has, in particular, improved delayed fracture resistance and toughness, for building and construction uses, and to a stainless steel screw, for example.
Conventional high strength and high corrosion resistance stainless steel screws made of martensitic stainless steel have high strength and low toughness in the center portion and are prone to generate screw head fracture caused by delayed fracture and the like.
The addition of Ni has been proposed as a measure to improve the toughness and the delayed fracture resistance of martensitic stainless steels (see Japanese Unexamined Patent Publication No. H9-206792).
On the other hand, a dual phase steel the outermost layer of which consists of martensite and the center portion of which consists of martensite and ferrite is known to be good both in ductility and strength (see Japanese Unexamined Patent Publication No. H7-316740).
The above technologies can improve the toughness and delayed fracture property of conventional stainless steels, but sufficient effects cannot always be obtained when they are applied to screws for high fastening strength use.
In view of the above situation, the object of the present invention is to solve the problems and provide, at low cost, a stainless steel having improved toughness and delayed fracture resistance, in addition to corrosion resistance and strength.
The inventors of the present invention discovered, as a result of various studies to solve the above problems, that it was possible to stably produce a high strength and high toughness stainless steel excellent in delayed fracture resistance by controlling the metallographic structure (martensite+austenite) at the surface of a dual phase stainless steel material through the control of its chemical composition and of surface reforming such as nitriding.
They also discovered that it was possible to stably produce a high strength and high toughness stainless steel excellent in delayed fracture resistance by accelerating the surface nitriding through structure control to make it easier to harden the surface and by lowering the hardness of the center portion. The present invention has been established based on these findings.
The first present invention is, therefore, a high strength and high toughness stainless steel-excellent in delayed fracture resistance comprising 11.0 to 16.0 mass % of Cr and characterized by having a mixed structure consisting of martensite and 3 to 30% of austenite in the surface layer from the outermost surface to the depth of at least 1 xcexcm.
The second present invention is a high strength and high toughness stainless steel excellent in delayed fracture resistance according to the first present invention, characterized in that said stainless steel comprises, by mass %, 0.06 to 0.25% of C, 0.05 to 1.0% of Si, 0.1 to 2.0% of Mn, 0.1 to 3.0% of Ni, 11.0 to 16.0% of Cr, 0.01 to 0.15% of N, and 0.01 to 3.0% of Mo, with the balance consisting of Fe and unavoidable impurities, and has less than 10% of ferrite structure in the center portion of the material.
The third present invention is a high strength and high toughness stainless steel excellent in delayed fracture resistance according to the first present invention, characterized in that said stainless steel comprises, by mass %, 0.01% or more but less than 0.06% of C, 0.05 to 1.0% of Si, 0.1 to 2.0% of Mn, 0.1 to 3.0% of Ni, 11.0 to 16.0% of Cr, 0.01 to 0.15% of N, and 0.01 to 3.0% of Mo, with the balance consisting of Fe and unavoidable impurities, and has 10 to 80% of ferrite structure in the center portion of the material.
The fourth present invention is a high strength and high toughness stainless steel excellent in delayed fracture resistance, as described above, characterized by containing 0.001 to 0.005 mass % of B.
The fifth present invention is a high strength and high toughness stainless steel excellent in delayed fracture resistance, as described above, characterized by containing, by mass %, 0.5% or less in total of one or more of 0.05 to 0.5% of Ti, 0.05 to 0.5% of Nb, and 0.05 to 0.5% of W.
The sixth present invention is a high strength and high toughness stainless steel excellent in delayed fracture resistance, as described above, characterized by containing 0.4 to 2.0 mass % of Cu.
Further, the seventh present invention is a method to produce a high strength and high toughness stainless steel excellent in delayed fracture resistance, characterized by nitriding a steel having the chemical composition described above in the temperature range equal to or higher than 950xc2x0 C. so as to form a mixed structure consisting of martensite and 3 to 30% of austenite in the surface layer from the outermost surface to the depth of at least 1 xcexcm.
The eighth present invention is a high strength and high toughness stainless steel screw excellent in delayed fracture resistance, characterized by: consisting of a steel having the chemical composition described above; having a mixed structure consisting of martensite and 3 to 30% of austenite in the surface layer from its outermost surface to the depth of at least 1 xcexcm; and having a surface hardness equal to or higher than Hv 450.
The ninth present invention is a method to produce a high strength and high toughness stainless steel screw excellent in delayed fracture resistance, characterized by nitriding a screw having the chemical composition described above in the temperature range equal to or higher than 950xc2x0 C. so as to form a mixed structure consisting of martensite and 3 to 30% of austenite in the surface layer from the outermost surface to the depth of at least 1 xcexcm.